1. Field of the Invention
The present invention relates to a molten steel pouring nozzle which permits effective prevention of reduction or clogging of a bore of the nozzle, through which molten steel flows, when continuously casting an aluminum-killed molten steel containing aluminum.
2. Related Art Statement
A continuous casting of molten steel is carried out, for example, by pouring molten steel received from a ladle into a tundish, through a molten steel pouring nozzle secured to a bottom wall of the tundish, into a vertical mold arranged below the molten steel pouring nozzle, to form a cast steel strand, and continuously withdrawing the thus formed cast steel strand into a long strand.
As the above-mentioned molten steel pouring nozzle, a nozzle comprising an alumina-graphite refractory is widely used in general.
However, the molten steel pouring nozzle comprising an alumina-graphite refractory has the following problems:
When casting an aluminum-killed molten steel, aluminum added as a deoxidizer into molten steel reacts with oxygen present in molten steel to produce non-metallic inclusions such as .alpha.-alumina. The thus produced non-metallic inclusions such as .alpha.-alumina adhere and accumulate onto the surface of the bore of the molten steel pouring nozzle, through which molten steel flows, to clog up the bore, thus making it difficult to achieve a stable casting for long period of time. Furthermore, the non-metallic inclusions such as .alpha.-alumina, thus accumulated onto the surface of the bore, peel off or fall down, and are entangled into the cast steel strand, thus degrading the quality of the cast steel strand.
For the purpose of preventing the above-mentioned reduction or clogging of the bore of the molten steel pouring nozzle caused by the non-metallic inclusions such as .alpha.-alumina present in molten steel, there is popularly used a method which comprises ejecting an inert gas from the surface of the bore of the molten steel pouring nozzle toward molten steel flowing through the bore, to prevent the non-metallic inclusions such as .alpha.-alumina present in molten steel from adhering and accumulating onto the surface of the bore.
However, the above-mentioned method, in which an inert gas is ejected from the surface of the bore of the molten steel pouring nozzle toward molten steel flowing through the bore, has the following problems:
A larger amount of the ejected inert gas causes entanglement of bubbles produced by the inert gas into the cast steel strand, resulting in the production of defects such as pinholes in a steel product after the completion of rolling. This problem is particularly serious in the casting of molten steel for a high-quality thin steel sheet. A smaller amount of the ejected inert gas causes, on the other hand, adhesion and accumulation of the non-metallic inclusions such as .alpha.-alumina onto the surface of the bore of the molten steel pouring nozzle, thus causing reduction or clogging of the bore. In the casting of molten steel for a long period of time, a stable control of the amount of the ejected inert gas from the surface of the bore of the molten steel pouring nozzle becomes gradually more difficult, according as a structure of the refractory forming the molten steel pouring nozzle degrades As a result, the non-metallic inclusions such as .alpha.-alumina adhere and accumulate onto the surface of the bore of the molten steel pouring nozzle, thus causing reduction or clogging of the bore. Furthermore, in the casting of molten steel for a long period of time, a local erosion of the surface of the bore of the molten steel pouring nozzle is considerably accelerated by the ejected inert gas. This makes it impossible to continue the ejection of the inert gas and may cause rapid clogging of the bore.
With a view to preventing reduction or clogging of the bore of the molten steel pouring nozzle without the use of a mechanical means such as the ejection of an inert gas as described above, there is disclosed in Japanese Patent Provisional Publication No. 64-40,154 published on Feb. 10, 1989, a molten steel pouring nozzle formed of a refractory consisting essentially of:
______________________________________ graphite: from 10 to 40 wt. %, calcium zirconate: from 60 to 90 wt. %, where, a content of calcium oxide in said calcium zirconate being within a range of from 23 to 36 weight parts relative to 100 weight parts of said calcium zirconate. (hereinafter referred to as the "prior art 1"). ______________________________________
where, a content of calcium oxide in said calcium zirconate being within a range of from 23 to 36 weight parts relative to 100 weight parts of said calcium zirconate. (hereinafter referred to as the "prior art 1").
However, the above-mentioned molten steel pouring nozzle of the piror art 1 has the following problems:
Calcium oxide (CaO) rapidly reacts with non-metallic inclusions such as .alpha.-alumina, which are produced through the reaction of aluminum added as a deoxidizer with oxygen present in molten steel, to produce low-melting-point compounds. Calcium oxide has therefore a function of preventing the non-metallic inclusions such as .alpha.-alumina from adhering and accumulating onto the surface of the bore of the nozzle.
However, calcium oxide, when present alone, violently reacts with water or moisture in the air even at a room temperature to produce calcium hydroxide (Ca(OH).sub.2), which easily disintegrates and tends to become powdery, thus leading to easy degradation of the structure of the molten steel pouring nozzle. Careful attention is therefore required for storing the molten steel pouring nozzle. Furthermore, because of a large thermal expansion coefficient of calcium oxide, a considerable thermal stress is produced in the interior of the molten steel pouring nozzle when calcium oxide is present alone and subjected to heating to such an extent as to cause a non-uniform temperature distribution, thus degrading thermal shock resistance of the molten steel pouring nozzle.
For the problems as described above, it is difficult to use the molten steel pouring nozzle made of a refractory, in which calcium oxide is present alone, for a long period of time for the continuous casting of molten steel.
For the purpose of overcoming the above-mentioned problems encountered in the molten steel pouring nozzle, in which calcium oxide is present alone, the molten steel pouring nozzle of the prior art 1 is formed of a refractory mainly comprising calcium zirconate. Therefore, it is true that contact of calcium oxide contained in calcium zirconate with the produced non-metallic inclusions such as .alpha.-alumina causes the acceleration of reaction between these components, thus producing low-melting-point compounds. Since calcium oxide is not present alone, no degradation of the structure of the molten steel pouring nozzle is caused. In the prior art 1, however, calcium oxide contained in calcium zirconate does not sufficiently move toward the surface of the bore of the molten steel pouring nozzle, through which molten steel flows, so that calcium oxide does not come into sufficient contact with the produced non-metallic inclusions such as .alpha.-alumina. As a result, the production of low-melting-point compounds caused by the reaction between calcium oxide and the non-metallic inclusions such as .alpha.-alumina is insufficient. Therefore, it is impossible to effectively prevent adhesion and accumulation of the non-metallic inclusions such as .alpha.-alumina onto the surface of the bore of the molten steel pouring nozzle.
Furthermore, with a view to preventing reduction or clogging of the bore of the molten steel pouring nozzle without the use of a mechanical means such as the ejection of an inert gas, there is disclosed in Japanese Patent Provisional Publication No. 3-221,249 published on Sep. 30, 1991, which corresponds to the U.S. Pat. No. 5,086,957 granted on Feb. 11, 1991, another molten steel pouring nozzle formed of a refractory consisting essentially of:
______________________________________ zirconia clinker comprising from 40 to 89 wt. %, calcium zirconate: where, a content of calcium oxide in said zirconia clinker being within a range of from 8 to 35 weight parts relative to 100 weight parts of said zirconia clinker; graphite: from 10 to 35 wt. %, and calcium metasilicate from 1 to 25 wt. %, (CaO.SiO.sub.2): where, a content of calcium oxide in said calcium metasilicate being within a range of from 40 to 54 weight parts relative to 100 weight parts of said calcium metasilicate. (hereinafter referred to as the "prior art 2"). ______________________________________
However, the above-mentioned molten steel pouring nozzle of the prior art 2 has the following problems:
It is true that calcium oxide (CaO) contained in calcium metasilicate (CaO.SiO.sub.2) never violently reacts with water or moisture in the air. Furthermore, when the zirconia clinker comprising calcium zirconate coexists with calcium metasilicate (CaO.SiO.sub.2), calcium oxide in each particle of the zirconia clinker tends to easily move toward the surface of each particle of the zirconia clinker under the effect of the coexisting calcium metasilicate (CaO.SiO.sub.2). As a result, calcium oxide rapidly reacts with non-metallic inclusions such as .alpha.-alumina contained in molten steel to produce low-melting point compounds, thus preventing reduction or clogging of the bore of the nozzle.
However, because of the low content of calcium oxide, calcium metasilicate (CaO.SiO.sub.2) cannot sufficiently replenish calcium oxide which reacts with the non-metallic inclusions such as .alpha.-alumina in molten steel, thus making it impossible to prevent reduction or clogging of the bore of the nozzle for a long period of time. If calcium metasilicate (CaO.SiO.sub.2) is added to the refractory in a large quantity to increase the content of calcium oxide, on the other hand, the high contents of impurities contained in calcium metasilicate (CaO.SiO.sub.2) causes degradation of spalling resistance of the molten steel pouring nozzle.
Under such circumstances, there is a strong demand for the development of a molten steel pouring nozzle which permits prevention of reduction or clogging of the bore of the nozzle and degradation of the structure of the refractory forming the nozzle economically and for a long period of time without the use of a mechanical means such as the ejection of an inert gas, but such a molten steel pouring nozzle has not as yet been proposed.